This is an excerpt from Happy Runner , The by David Roche & Megan Roche.
Nearly every runner who probes the limits of their genetic potential has a story to tell about the "Trial of Miles."
The Trial of Miles is the superhero origin story of countless runners. In John L. Parker Jr.'s cult classic novel Once a Runner, the protagonist Quenton Cassidy is expelled from college, moves to the woods, and simply begins running lots. As described by his spiritual mentor in the book: "The only true way [to maximize your running potential] is to marshal the ferocity of your ambition over the course of many days, weeks, months, and (if you could finally come to accept it) years."
As Cassidy learns, the secret of running training is simple: it's the "process of removing, molecule by molecule, the very tough rubber that comprised the bottoms of his training shoes."
Spoiler alert! That book ends with Cassidy winning an Olympic medal. But the main theme is that what comes from the Trial of Miles is beyond the point. The Trial of Miles is the point.
When you think about it, every complex skill in life works similarly. You never read about a well-balanced piano prodigy with a flourishing social life. You don't want a surgeon who inconsistently wields a scalpel. Running is the same. Practice might not make perfect, but it makes better. And lots of better over time will eventually get you as close to perfect as you can possibly get.
What separates running from the piano and surgery is the potential for physical breakdown. Heck, that is also what separates running from other endurance sports like swimming and biking. In running, each step involves a big impact. All of those impacts add up to a lot of pounding on the body over time. In cycling and swimming, meanwhile, the body doesn't bear the weight of impact and you can do a lot more before the point of no return. The most a runner will train (outside of outliers) is 10 to 15 hours a week consistently. Some cyclists and swimmers train upward of 30. Surgeons might be up over 100. There's a reason medical training involves something called "residency"—historically, they expected young doctors to live at the hospital.
Even at a relatively measly 10 hours a week (when compared to other sports), running is risky unless you do it strategically. You have 206 bones, and lifelong runners can learn a disturbing number of them from stress fracture injury scares. If a runner Google searches their symptoms enough, they could probably pass a med school anatomy test after a few years. Or, at the very least, they could ruin anatomy nursery rhymes. "The hip bone is connected to the...femoral neck, which I learned from doing too many track workouts in 2008."
The key to running development is balancing risk of injury and burnout with your own personal Trial of Miles. How does it all work? It's all about a progressive series of adaptations that happen as you run more over time. Disclaimer: These are complex physiological processes that could each take up 10,000 words and be the subject of an entire episode of The Magic School Bus. So we will gloss over some of the details for the sake of not making you get really sleepy and involuntarily snoozle like Addie dog with your snout in the spine of the book.
First, there's aerobic development, primarily via the cardiovascular system. As you run more, your body improves oxygen processing capacity and your ability to handle fatigue. At the easy end of the spectrum, you develop more capillaries and mitochondria that supply and process fuel for working muscles. Capillary growth is called angiogenesis, and recent studies indicate that angiogenesis is impaired by too much intensity. While the hare focused on impressing its training partners, the tortoise must have been studying exercise physiology.
At moderate intensities, your body gets better at using fat for fuel at faster paces, which means you can go farther and more efficiently before bonking. All these adaptations support harder workouts that improve lactate threshold (essentially how well you buffer against fatigue-causing chemical byproducts of intense exercise) and VO2 max (raising your aerobic capacity). It's like a 100-story skyscraper—the first 80 stories are low-level aerobic development, and that allows the top floors to reach higher.
Bones, joints, and tendons get stronger as you run more too. The best example of what happens when you start running may be from the TV show Parks and Recreation, where the lovable Andy Dwyer tries running for the first time. After one lap around the track, he stops and begins to take off all of his clothes. "This is horrible! Ugh, I'm going to die! I'm so tired, everything hurts!" And when he finally gets all of his clothes off, he lays down on the track in his underwear. "Running is impossible!"
What Andy would have learned with consistent running is that it starts out impossible because the body first needs to adapt to the mechanical load. Repetitive bouts of low-level stress provide a stimulus that strengthens the musculoskeletal system to handle more impact. Running plus adequate recovery promotes development of osteoblasts, which become bone cells called osteocytes, which become stronger bones. After stress and recovery, the body repairs damaged muscle fibers to become new muscle strands called myofibrils that are stronger than before. A similar process can occur in tendons and ligaments. Meanwhile, enzymatic activity and angiogenesis spur aerobic development. Essentially, the body makes itself anew to respond to repeated bouts of low-level stress. That's why frequency and consistency are so important for runners, and why working up to five or six shorter runs a week is usually better than three longer runs.
At the same time, when you run more, your muscle fibers can actually change their properties altogether to become progressively better equipped for endurance exercise. We all have a genetic predisposition to certain types of muscle fibers, generally broken down into fast-twitch (FT) and slow-twitch (ST). ST fibers are full of mitochondria and capillaries, using oxygen and built to withstand fatigue for longer distances. FT fibers generate more force, but it comes with a catch—they get tired quickly. Type IIx FT fibers use carbohydrate fuel and are anaerobic, meaning they can only go for a short time. A good example is a swimmer competing in the 50-meter sprint at the Olympics. They are recruiting type IIx FT fibers, so some can go the entire distance without taking a breath at all. Meanwhile, type IIa FT fibers are intermediate, using aerobic energy sources in a generally more sustainable way.
Consistent aerobic training makes a runner more ST (and type IIa FT) than their genetic predisposition. That sounds weird, right? Don't you want fast-twitch to go fast? David's experience shows why hard-running FT fibers are not optimal for endurance running.
After he graduated high school, David was a protein shake-based organism. He was under six feet tall and weighed 200 pounds of thick muscle, as if a pot roast was granted its wish of becoming a real boy. His training consisted of explosive movements like sprinting and weightlifting.
As a result, he was a good sprinter and mediocre football player, landing himself a spot on the Columbia football team. Now, stop right there if you think that's impressive. At the time, Columbia football was one of the worst sports teams in the country, like an opposite-world Harlem Globetrotters, getting dunked on repetitively by opponents in increasingly humorous fashion. Still, David couldn't cut it for the Washington Generals of college football. So he was left with lots of FT muscle fibers and nothing to do with them unless someone needed help moving their couch.
His dad had always been an endurance athlete, and David had run as a kid, so he chose his next adventure. He was going to be an endurance runner. Don Quixote would have been proud of his hubris. What would happen when a sprinter chased the windmill of endurance running?
The results were predictably Quixotic. Six months after quitting football and not running a step in the interim, he laced up his lifting shoes and ran out the door; 300 meters later, he stopped, winded and sore.
That sounds like an exaggeration, but it's true. David literally got less than a quarter of a mile on his first endurance run as an adult. Most embarrassingly, it left him sore for days. We are talking "can't walk in a straight line" sore. We are talking "calf muscles lose all ability to function" sore. To paraphrase Andy Dwyer, running was impossible.
Little by little, David's body adapted. Over the next few years, he'd lose 60 pounds of muscle and fat. The pot roast gradually became a veggie burger. The changes were all small and gradual, happening at the cellular level. He probably got a bit more ST-oriented as muscle fibers adapted over time. His joints and tendons adapted to the impact forces. He no longer felt like he was breathing through a crazy straw when jogging as his aerobic system became less feeble.
But perhaps the biggest change wasn't in the muscles, but in his understanding of running. He read every book there was to read and constantly listened to running mentors to learn what he missed by not having teams in high school or college. And he came upon the same epiphany as Quenton Cassidy. It's all about the Trial of Miles; Miles of Trials.